As portable compact electronics such as cell phones, personal digital assistances (PDAs), notebook computers and camcorders perform more functions, they consume more electric power and operate for a longer period of time. In order to satisfy such increasing power demand and to achieve longer continuous operation, batteries for portable compact electronics having a higher energy density are in strong demand.
Currently, lithium secondary batteries are widely used as the main power supply for portable compact electronics. Lithium secondary batteries are expected to have an energy density of about 600 Wh/L by 2006, which is considered as the maximum energy density for lithium secondary batteries. As an alternative to lithium secondary batteries, early commercialization of fuel cells having a solid polymer electrolyte membrane is eagerly awaited.
Among fuel cells, direct methanol fuel cells (DMFCs) are attracting attention and being vigorously studied in which a fuel such as methanol or an aqueous solution of methanol is fed directly into the fuel cell for power generation without converting the fuel into hydrogen. This is because methanol has a very high theoretical energy density and offers advantages of simple system design and easy storage.
A single unit cell contained in a direct methanol fuel cell comprises a membrane electrode assembly (MEA) and separators disposed on both sides of the MEA. The membrane electrode assembly (MEA) comprises a solid polymer electrolyte membrane, an anode attached to one surface of the solid polymer electrolyte membrane, and a cathode attached to the other surface of the solid polymer electrolyte membrane. The anode and the cathode each comprise a catalyst layer and a diffusion layer.
A direct methanol fuel cell generates electricity (power) by feeding a fuel (i.e., methanol or an aqueous solution of methanol) directly into the anode and air to the cathode. In the direct methanol fuel cell, the following reaction occurs.Anode: CH3OH+H2O→CO2+6H++6e−Cathode: 3/2O2+6H++6e−→3H2O
That is, methanol reacts with water at the anode to produce carbon dioxide, protons and electrons. The protons pass through the electrolyte membrane to reach the cathode. At the cathode, oxygen combines with the protons and electrons migrated into the cathode through an external circuit to produce water.
In order to achieve commercialization of direct methanol fuel cells, however, the following problem must be solved.
Direct methanol fuel cells employ, as the electrolyte membrane, a perfluoroalkyl sulfonic acid membrane from the viewpoint of proton conductivity, thermal resistance and resistance to oxidation. Electrolyte membranes of this type comprise a main chain of hydrophobic polytetrafluoroethylene (PTFE) and a side chain of a perfluoro group having hydrophilic sulfonic acid group fixed at the terminal of the perfluoro group. Accordingly, methanol having both hydrophilic and hydrophobic parts is a suitable solvent for a perfluoroalkyl sulfonic acid membrane because methanol can easily pass through the polymer electrolyte membrane. However, a phenomenon called “methanol crossover” occurs in which methanol fed into the anode pass through the electrolyte membrane to the cathode, without reacting. This methanol crossover not only reduces the fuel utilization efficiency but also the potential of the cathode, which significantly degrades power generation characteristic. This methanol crossover tends to increase as the methanol concentration and the operating temperature get higher.
In order to reduce methanol crossover, in addition to the development of novel electrolyte membranes, various proposals are made to modify the structure of anodes.
For example, in order to reduce methanol crossover at the upstream side of a fuel flow channel and to prevent supply shortage of methanol at the downstream side of the fuel flow channel so as to uniformly supply a fuel to the anode, Japanese Laid-Open Patent Publication No. 2002-110191 proposes a structure in which the methanol permeability coefficient of an anode diffusion layer is higher at the downstream side of the fuel flow channel. The anode diffusion layer disclosed in Japanese Laid-Open Patent Publication No. 2002-110191 comprises a carbon paper substrate and a mixed layer containing carbon black and polytetrafluoroethylene formed on a surface of the substrate. In order to increase the methanol permeability coefficient of the anode diffusion layer along the fuel flow channel, the publication further discloses to reduce the thickness of the mixed layer, to reduce the weight ratio of the polytetrafluoroethylene, to reduce the water repellency of the carbon black, and to increase the porosity and/or pore size of the carbon black.
Another problem that must be solved to achieve commercialization of direct methanol fuel cells is removability of carbon dioxide (CO2) gas produced at the anode from the anode diffusion layer. In other words, the carbon dioxide gas permeability of anode diffusion layer should be improved. The carbon dioxide gas produced at the anode passes through the anode diffusion layer, reaches the flow channel of a separator and then to the outside. If the anode diffusion layer has low gas permeability, some of the generated gas might accumulate inside the diffusion layer, inhibiting the dispersion of fuel into the catalyst layer. Furthermore, the carbon dioxide gas might gradually coalesce to form large bubbles, which squeeze the fuel from the pores of the diffusion layer, reducing the supply amount of fuel to the anode catalyst layer. As a result, power generation characteristic at a high current density, which requires a large amount of fuel, decreases significantly.
In order to overcome this problem, Japanese Laid-Open Patent Publication No. 2002-175817 proposes to provide separate (completely segregated) flow channels for liquid fuel and exhaust gas on an anode-side plate (i.e., separator), and to impart liquid permeability and gas impermeability to the area of an anode diffusion layer that faces the flow channel for liquid fuel, as well as gas permeability to the area of the diffusion layer that faces the flow channel for exhaust gas.
Such conventional structures as described above, however, cannot provide a direct methanol fuel cell having excellent power generation characteristic without impairing fuel utilization efficiency. Many problems still remain.
Japanese Laid-Open Patent Publication No. 2002-110191 fails to give adequate consideration to the methanol permeability coefficient of the anode diffusion layer. That is, the effect of methanol concentration, the effect of operating temperature (temperature for power generation) and the balance between the electrolyte membrane and the methanol permeability coefficient are not fully discussed. The fuel cell disclosed by the above publication thus suffers from the problem that the power generation characteristic degrades significantly at a high current density in the case of, for example, using highly concentrated methanol or increasing the operating temperature (temperature for power generation).
As for the fuel cell disclosed by Japanese Laid-Open Patent Publication No. 2002-175817, there is a prospect that the problems such as the supply of a sufficient amount of fuel and the removal of carbon dioxide will be solved. The publication, however, fails to disclose any solution for methanol crossover.
In view of the foregoing, an object of the present invention is to provide a direct methanol fuel cell having excellent power generation characteristic without impairing fuel utilization efficiency by reducing methanol crossover and ensuring that a sufficient amount of fuel is supplied to the catalyst layer. Another object of the present invention is to provide a direct methanol fuel cell having excellent power generation characteristic by also improving the removability of carbon dioxide from the anode catalyst layer.